International Journal of Computer Applications (0975 – 8887) Volume 141 – No.8, May 2016 Numerical Analysis of the Effects of Soil Nail on Slope Stability

Md. Akhtar Hossain, PhD Ashraful Islam Associate professor Department of Civil Engineering Department of Civil Engineering Rajshahi University of Engineering & Technology, Rajshahi University of Engineering & Technology, Rajshahi Rajshahi

ABSTRACT soft rocks. Unlike the conventional systems that serve to Soil nailing is one of the good techniques for the improvement retain soil behind a vertical cut, this procedure is based on the of natural and artificial slope. By the application of soil nails concept of soil reinforcement. The concept of soil nailing is to the stability can be increased of an unstable slope. The reinforce the ground by closely spaced nails to increase the appropriate position of reinforcement can give the ultimate overall shear strength of soil. There are usually using steel bar result, and it will be solution of reducing cost. In this paper, inserted into the soil either by simple driving or by grouting in an attempt has been made to show the stability of a reinforced predrilled borehole. Soil nail slopes behave like a reinforced dry slope at different nail angles with horizontal axis, and soil wall although there is some major difference between determine the optimum nail inclination to get maximum factor these two techniques. The action of grouting the of safety. The analysis was based on numerical analysis by reinforcement leads to the advantage of greatly enhanced using SLOPE/W (Geo-slope 2007). The factor of safety has pullout resistance because of the increased surface area and been determined using the limit equilibrium (LE) method roughness. The presence of grout may also provide increased within the Morganstern-Price method along with Mohr- corrosion protection [2]. Coulomb expression. The results show that the factor of safety Presently, stability of slope can be analyzed using several (FOS) of slope increases with the application of soil nail. The geotechnical software. GEOSLOPE is one them developed results also show that with the increase of inclination the by GEOSLOPE International, Canada, based on limit factor of safety of the slope increases, but after reaching the equilibrium principles which also incorporates a finite maximum value further increases of inclination it decreases. element package, developed specifically for the analysis ο The optimum angle of nail was found at 30 with the of deformation and stability of geotechnical structures. It horizontal. includes stability modeling (SLOPE/W), seepage modeling (SEEP/W), stress and deformation modeling (SIGMA/W), Keywords dynamic modeling (QUAKE/W), thermal modeling Slope stability, Factor of safety, Soil nail, Numerical (TEMP/W), containment modeling (CTRAN/W) and vadose Analysis, Limit Equilibrium Method. zone modeling (VADOSE/W) [3]. SLOPE/W is a component of a complete suite of geotechnical products called Geo 1. INTRODUCTION Studio. SLOPE/W uses limit equilibrium theory to compute The slope stability analyses are performed to assess the safe the factor of safety of earth and rock slopes. In the present and economic design of human-made or natural slopes (e.g. study, SLOPE/W module has been used for stability modeling embankments, road cuts, open-pit mining, excavation, and of the slope. The comprehensive formulation of SLOPE/W landfills). In assessment of slopes, engineers primarily use makes it possible to easily analyze both simple and complex factor of safety (FOS) values to determine how close or far slope stability problems using a variety of methods to slopes are from failure. When FOS is greater than 1, resistive calculate the factor of safety. SLOPE/W has application in the shear strength is greater than driving shear stress and the slope analysis and design for Geotechnical, Civil, and Mining is considered stable. When FOS close to 1, shear strength is engineering projects. Limit equilibrium has the ability to nearly equal to shear stress and the slope is close to failure, if model heterogeneous soil types, complex stratigraphic and FOS is less than 1 the slope should have already failed [1], slip surface geometry, and variable pore water pressure Thus, there are various process to improve the slope stability, conditions using a large selection of soil models [2]. soil nailing is one of them. In this paper an attempt has been made to show the effects of variation of nail inclination on SLOPE/W can be used to analyze the stability of a wedge of stability of a dry slope, and determine the optimum nail soil that has been reinforced with a structural component such inclination for maximum FOS. as a pre-stressed anchor, a soil nail, geo-fabric or some other material. While modeling using, SLOPE/W, the following are Soil nailing is an in-situ soil reinforcement technique, which mainly required to be satisfied has been used during the last three decades. This technology was first reported to apply for the permanent support of  Type of analysis: Bishop, Ordinary and Janbu, retaining walls in a cut in soft rock in France in 1961. Besides, Morganstern-Price. in North America, soil nails were first introduced for temporary excavation support in Vancouver, in the late 1960’s  Strength parameters: c and φ, unit weight of soil γ. and early 1970’s. It was continued to grow in the 1970’s, in  •Piezometric line while considering pore water France and Germany. pressure conditions. Soil nailing is being used at present to stabilize natural slopes,  •Vertical and horizontal seismic coefficient for cut or excavation, walls in stiff clays, granular soils and also pseudo-static seismic loads.

12 International Journal of Computer Applications (0975 – 8887) Volume 141 – No.8, May 2016

 Type of reinforcement: Soil nails, Anchors, Geo- Table 2. Nail properties [5] fabric [4]. Property value 2. METHODOLOGY In this study SLOPE/W has been applied to analyze the stability of the slope. The CSS searching techniques Entry- Length 10m exit method is used. The most common LE based method (M- PM), which is incorporated in SLOPE/W, was considered for Bond diameter 0.318m analysis purpose. 2.1 Stability analysis of a dry slope without Bond safety factor 1.5 nail Bond skin friction(F/Area) 100 kPa In the analysis of slope stability the slope model is drawn using SLOPE/W. The physical properties of the soil were considered as described in the Table 1. The slope angle is 45ο Bar capacity 300 kN (1V:1H). The factor of safety of the slope without nail is 1.340. Bar safety factor 1.5 Table 1. The soil properties for each layer of the slope Nail spacing 2m Cohesion, Friction Unit Weight, Layer c (kPa) Angle, φ(ᵒ) γ (kN/m3) Shear safety 1

Upper soil 25 12 17 layer

Middle soil 20 16 16 layer

Lower soil 30 10 19 layer

(a)

Fig 1: Slope stability analysis of a dry slope without soil nail 2.2 Stability analysis of a dry slope at various nail inclination (b) The geometry of the dry slope discussed in the earlier section is considered here. The stability is analyzed for different inclination of nail as 20ο, 25ο, 30ο, 35ο, 40ο, 45ο in the slope surface with the horizontal axis.

13 International Journal of Computer Applications (0975 – 8887) Volume 141 – No.8, May 2016

Figure 3 shows the variation of factor of safety for various nail inclination

1.65

1.583 1.586 1.558

1.6 1.558 1.531

1.55 1.499 1.5 1.45

FOS 1.4 1.35 1.34 1.3 1.25 (c) 1.2

Nail inclination (deg.)

Fig 3: Diagram of factor of safety of dry slope for various inclination of nail It is observed that within the use of nail the factor of safety increases. It is also observed that with the increase of nail ο (d) inclination the factor of safety increases, and at 30 it gives the maximum factor of safety and after that for increasing nail inclination the factor of safety decrease. 3. CONCLUTIONS The analyses were carried out to evaluate the effects of variation of nail inclination on dry slope. From the above investigation, the main conclusions can be summarized as follows: • The study of slope stability problems by using the computer based geotechnical software code SLOPE/W provides more understanding viewing all the detailed force on each slice, to understand failure mechanisms, and the distribution of variety of parameters along the slip surface with respect to (e) the factor of safety. • The factor of safety of a slope increase with the use of nail. • • From the result of analysis, it is found that the variation of nail inclination have significant influence on the factor of safety of the slope stability problem. • The factor of safety of a dry slope increases with the increase of nail inclination, but after a certain inclination of nail it decreases. • The optimum nail inclination improves the factor of safety by 18%. 4. REFERENCES (f) [1] Rouaiguia, A. and Dahim, M. A. 2013. Numerical Modeling of Slope Stability Analysis. International Fig 2: Shows the factor of safety of dry slope at different ο ο ο ο ο Journal of Engineering Science and Innovative nail inclination (a) 20 , (b) 25 , (c) 30 , (d) 35 , (e) 40 , (f) Technology. 2(3), 533-542. 45ο

14 International Journal of Computer Applications (0975 – 8887) Volume 141 – No.8, May 2016

[2] Chin, T. C. 2005. Application of soil nailing as a 2012. Proceeding published by International Journal of protection and reinforcement for slope stabilization. Computer Applications (IJCA).29-32. Fakulti Kejuruteraan Awam University Teknologi Malaysia. 27-35. [4] Sakia, R., Deka, P., Kalita, S. R. nad Dey, A. 2014. Analysis and behavior of hill slopes and their [3] Ghosh, P. and Biswas, a. 2012. Effect of stabilization measures. Proceedings of Indian Reinforcement on Stability of Slope using Geotechnical Coonfernces IGC-2014.2183-2190. GEOSLOPE. Intenational Conference on Benchmarks in Engineering Science and Technology ICBEST [5] SLOPE/W. An Engineering Methodology. Geo-Slope. International, Calgary, Canada, 2007.

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